Electrostatic printing



July 14, 1964 K. A. METCALFE ETAL 3,140,945

ELECTROSTATIC PRINTING Filed June 8, 1960 y* 2 Sheets-Sheet 1 2 Shee's--SheefI 2 K. A. METCALFE ETAL ELEcTRosTATIc PRINTING July 14, 1964 Filed June 8, 1960 4 4 w @E 96 7 W l fw 0W. E 2 ...m 1 fw/ @N m m I mw/ a 1 M W I j Fn 5 7 A Bm 9 Z 5 OW 6 W w y zw 6M n 5 o w 4M W w m y v, Q 5D A 5 M 3M w w w M U 1 A A 0 3 E 1.1 1./ o 6 7 6 5 4 a 20,6642@ 32 10.0.4 0 0 0 O O l 0 O 0 0 .tm Q MQ@ QSE@ m United States Patent O 3,140,945 ELECTROSTATIC PRINTING Kenneth Archibald Metcalfe, Graynlore, South Australia, and Robert John Wright, Hectorville, South Australia, Australia, assigiors to The Commonwealth of Australia, care of the Secretary of the Department of Supply, Melbourne, Victoria, Australia Filed June 8, 1960, Ser. No. 34,831 Claims priority, application Australia June 19, 1959 2 Claims. (Cl. 96-1) This invention relates to improvement in and relating to electrostatic printing, and in particular it relates to printing of the type where a surface is charged to retain an electrostatic image which is then rendered visible by applying a developer which is electrically selective to the lield pattern.

One method of producing an electrostatic picture in this way is to use a photoconductor surface which is charged by means of a corona device and then has the charge modified by applying a light pattern thereto, the leakage of the charge taking place in proportion to the amount of light striking any part of the surface.

Development of such an electrostatic pattern can then take place by subjecting the surface to a liquid developer comprising a carrier liquid of high electrical resistivity in which is suspended the ldeveloper medium. It is also known to use dry development by simply subjecting the pattern to a powdered developer. It will, however, be realized that the actual method of development, as well as the method of producing the electrostatic pattern itself, is not an essential to this invention, the invention applying to the improvement of xerographic printing of the general types already well known in the art.

In effecting printing by the known methods, it has been noted that there is a gradual diminution of the charge as the process proceeds, and we have ourselves used this, as set out in an earlier application, to control the picture by regulating the amount of bleed in relation to the time of development.

We have now found however that there is a further factor which tends to exert quite an appreciable influence on the successful printing of electrostatic images, and that is the bleeding away of the charge because of the electrical gradients which exist.

We have Afurther found that this bleeding away is particularly influenced by the surrounding areas of an electrostatic image, and appreciable flattening or loss of definition can take place due to bleeding away of the charge across the surface to uncharged areas.

It is the object of this invention to provide certain irnprovements to the methods used in electrostatic printing to remove the disability which exists where this form of bleeding away, or diminution of the charge pattern, takes place.

The objects are achieved by applying a charge pattern which counters electron flow from the pattern area to the surroundings.

This can be achieved in many ways but according to a convenient form of the invention, the area surrounding the picture itself is arranged to maintain a pattern charge so that it forms as it were an electrical fence which prevents outward ow.

Such a boundary area can simply be left in a charged condition when the picture is applied, resulting in a heavily developed area when development has been completed, or a boundary layer of the necessary charge value can be separately applied so that there is a lesser gradient from the picture to the surrounding areas, or a maintained field can be used which can be continuously generated during development to insure that the surrounding areas are maintained at a potential which discourages electron ilow from the picture itself into these areas.

3,140,945 Patented July 14, 1964 Where ordinary development is being carried out immediately after exposure, the effect referred to is of course not as marked as where there is a delay between the time of exposure of the image and the development of same, although there is a cleaning of the image even in such cases.

While the invention applies generally in immediate development as well as where there is a delay between the production of the electrostatic image and its development, a typical example of application of the invention is in electrophotography where normally a photo-conductor surface is exposed in a camera and subsequently developed.

In the ordinary Way there can be a considerable loss in the image under such conditions but as will be shown later herein, when the present invention is used, the losses are considerably minimized and also the contrast is better maintained.

So that the invention can be fully understood certain embodiments will now be described with reference to the drawings in which:

FIG. l illustrates how the invention may be applied to a film or paper having a photo-conductor surface, the picture area being shown surrounded by a fringe charge area,

FIG. 2 is a view similar to FIG. 1 but showing how a eld can be applied through a surrounding ring to which a potential is applied which is of the same sign as the xerographic charge and which maintains the fringe charge area independently of the photo-conductor surface,

FIG. 3 shows the invention being applied in a camera, the camera being somewhat diagrammatically illustrated in section, there being shown a magazine in which the xerographic paper is carried on which the picture is taken,

FIG. 4 shows a charging device which can conveniently be used with such a magazine,

FIG. 5 is a graph of reflection density of image deposit against time delay to show the advantages of using a fringe charge,

FIG. 6 is a similar graph giving the relative comparison between immediate development and a two-hour delay with a fringe and also a two-hour delay without a fringe, and

FIG. 7 is a graph giving a relation of exposure time to. reflection density with and without a fringe charge.

Referring iirst to FIG. l it will be seen that the picture area 1 is surrounded by a fringe charge area 2 which has approximately the same area as the picture area 1, it being noted that satisfactory results can be obtained with a much lower fringe area such as, for example, one half. In this case, ordinary xerographic paper such as zinc oxide coated paper is charged in the normal manner over the whole of its area but the picture is formed on only the inner area, namely the picture area 1, by bleeding away the image charge by the projection onto the surface of light through a negative or the like. The fringe area is shielded against light.

The full voltage is therefore maintained on the fringe charge area 2, whereas in the picture area 1 the average voltage is reduced due to the formation of the image.

It has been found that this then prevents the electrons from migrating outwardly from the picture area, this effect forming the basis of the invention.

In the case of FIG. 2 the picture area 3 is surrounded by a ring 4 to which a potential is applied by means such as the battery 5 which is connected between the ring 4 and a metal backing plate 6 onto which the xerographic paper 7 is placed. This then gives a field over the fringe area as in FIG. l and again migration of the electrons from the picture area is thereby discouraged. The poten- Referring to FIGS. 3 and 4, a camera 10 is providedv with the usual lens 11 and shutter mechanism 12 whereby an exposure of any required time can be effected.

On the rear of the body of the camera is a fiange 13 against which a magazine 14 can be attached, the magazine comprising a housingT 15 carrying within it the sheet of xerographic paper 16, the paper being normally protected against light by the customary slide 17 in the front of such magazine 14, which slide 17 is withdrawn as shown when the magazine 14 is in place on the camera 10 as shown in FIG. 3. Within the camera is a mask 13 which extends right around the window 19 which defines. the picture area. When a picture is taken, the lens 11 will cause the aera of the xerographic paper behind the window 19 to be light-modified in the usual way, but the marginal areas of the paper behind the mask 18 will not be so modified and will retain their full charge, a condition as exemplified in FIG. 1 being thus obtained.

After a picture has been taken, the slide 17 is positioned to close off the magazine 14 against light and the magazine can then be put aside until the image is to be developed, the fringe charge then having the effect of limit ing migration of electrons from the picture area during such storage time.

In FIG. 4 is shown a suitable device which charges the xerographic paper 16, this comprising a light-tight box 20 to the open face of which the magazine 14 can be attached in a manner similar t0 its engagement on the camera. The slide 17 is pulled out after the magazine 14 is positioned on the box 2f).

Within the box 20 is a bar 21 having a series of points 22 thereon, a corona discharge being formed between the points 22 and the metal back 23 of the magazine 14 by means of a high voltage generating device of any suitable type. In the illustration the device is indicated by the battery 24 connected between the bar 21 and the back 23 of the magazine 14. The high voltage device can of course, be of any of the usual types already known in the charging of xerographic papers.

By means of the apparatus outlined, the operator can charge the xerographic paper 16 in the magazine 14 and can then use such a magazine 14 in the camera 10 in the manner outlined. Between charging and exposure the paper is kept in the dark by pushing in the slide 17. Storage of the exposed papers can take place until such time as the operator can develop the image, the image being protected for a reasonable time by the fringe charge.

Some indication of the advantages obtained by the invention and the protection given may be obtained from the graphs designated FIGS. 5, 6 and 7 which will now be described.

The graph of FIG. 5 shows how the reflection density of the image deposit varies with the time delay between normal exposure and development.

The upper curve A shows the density using a fringe of the type shown in FIG. l, the lower curve B showing the loss of density due to there being no fringe.

To produce this graph, standard zinc oxide paper commonly used in xerographic work, consisting of zince oxide particles embedded in an insulating matrix, was first charged to produce a negative voltage of approximately 200 volts on the surface of the paper.

The charging action is a standard procedure using a corona discharge, and requires no further description herein.

The picture area was then exposed for three seconds using a 160 watt blue actinic lamp at 24 inches from the paper, the area around the image in the one case being shielded so that the initial voltage remained on the paper to form the fringe charge, the other paper having its edge unprotected so that the voltage was bled away from around the area of the image. The shielded area in relation to the image area was approximately in the ratio of 1:1.

Five sets of papers were made in this way at hourly intervals and the papers were all developed together under standard conditions at the end of a four-hourly period.

The developed images on the paper were then read on a Baldwin refiection densitometer and the result plotted as shown in FIG. 3.

From this it will be clear that the fringe charge had little effect on the density on the image which was developed immediately, but after a four-hour delay the fringe charge had maintained the voltage, and thus the density of the image, at a value of approximately twice that existing where no fringe charge had been used.

In FIG. 6 three sample papers were exposed through a standard step wedge (plotted as the transmission density) for four seconds under 160 watt blue actinic lamp at 24 inches.

One of thse papers was immediately developed and reliection densities of the image deposits, as controlled by the step wedge, were measured on the Baldwin reflection densitometer and were plotted against the transmission density of the original wedge.

Line C is the result of that plotting.

Similar plotting was carried out on the two other papers, one with the fringe charge and one without, but in this case they were not developed until after a twohour delay.

The line D shows the density in relation to the density of immediate development and it will be noted that excepting for the lower part of the line, the lines C and D are substantially parallel over the major portion of the density wedge readings.

The line E, however, which represents the condition where no fringe charge existed, shows that the density had materially decreased, showing of course that the voltage remaining on the paper had dropped.

As well as having a drop in the voltage, the line E falls throughout its length in relation to C and D showing that the contrast has also been reduced owing to a difference in bleeding away of the image.

From this it is apparent that the fringe charge is most important where development is delayed, as it tends to hold the electrostatic image nearer to its original value, and a much improved picture is therefore possible when using such a fringe charge.

FIG. 7 shows an interesting effect of fringe charges, namely, it shows that there is less background when using a fringe charge, the line F showing the background density where no fringe is used and the line G showing the background density when using a fringe.

To obtain this graph a series of pairs of xerographic papers were exposed for different periods of time, one paper being exposed to leave a fringe charge and one without.

The vertical figures represent the exposure in seconds using a watt lamp at 24 inches from the paper while the base line figures represent transmission densities of the developed images. Development was carried out immediately after exposure.

It was found in every case that where no fringe was present the background density was slightly higher, the effect possibly being due to the fringe (which has a relatively higher voltage) denuding the developer particles over the image area due to a relatively greater electrical gradient at the fringe, and therefore limiting the deposition on the image area.

This effect has been of particular interest in all fringe charge developments as the image is found to be cleaner with less background and better grading of the different densities.

From the foregoing it will be seen that the fringe charge effect will increase the cleanness of the print as well as maintaining the charge for a longer time where development delays take place, and also keeping the rate of decay more uniform throughout the range than where no fringe charge is used.

We claim:

1. An electrostatic reproduction method comprising forming, on a surface adapted to receive an electrostatic charge, an electrostatic image constituted by the selective positioning of electrostatic charges, and preserving said image by surrounding the same on said surface with an enclosure of a substantially uniform electrostatic charge adapted to repell the rst said charges and thus prevent the latter from leaking from said surface, said enclosure having an area of at least about one-half the area which is enclosed.

2. A method as claimed in claim 1 wherein said surface is the surface of a photoconductor sheet, comprising uniformly charging said surface preparatory to generating an electrostatic image and photographically bleeding away the uniform charge in selective manner to form the image while shielding a portion of the surface encircling the image to retain the uniform charge in said portion to create an electrostatic fence.

References Cited in the le of this patent UNITED STATES PATENTS 1,817,606 Bern Aug. 4, 1931 2,277,013 Carlson Mar. 17, 1942 2,295,000 Morse Sept. 8, 1942 2,693,416 Butterfield Nov. 2, 1954 2,803,177 Lowrie Aug. 20, 1957 3,009,402 Crumrine et al Nov. 2l, 1961 FOREIGN PATENTS 326,200 Great Britain 1930 OTHER REFERENCES Xerography Today, by Dressauer, Mott, and Bogdonoff, published in 1955, Photographic Engineer, vol. 6, No. 4, pages 250-268 (page 264 is being relied on). 

1. AN ELECTROSTATIC REPRODUCTION METHOD COMPRISING FORMING, ON A SURFACE ADAPTED TO RECIEVE AN ELECTROSTATIC CHARGE, AN ELECTROSTATIC IMAGE CONSTITUTED BY THE SELECTIVE POSITIONING OF ELECTROSTATIC CHARGES, AND PRESERVING SAID IMAGE BY SURROUNDING THE SAME ON SAID SURFACE WITH AN ENCLOSURE OF A SUBSTANTIALLY UNIFORM ELECTROSTATIC CHARGE ADAPTED TO REPELL THE FIRST SAID CHARGES AND THUS PREVENT THE LATTER FROM LEAKING FROM SAID SURFACE, SAID ENCLOSURE HAVING AN AREA OF AT LEAST ABOUT ONE-HALF THE AREA WHICH IS ENCLOSED.
 2. A METHOD AS CLAIMED IN CLAIM 1 WHEREIN SAID SURFACE IS THE SURFACE OF PHOTOCONDUCTOR SHEET, COMPRISING UNIFORMLY CHARGING SAID SURFACE PREPARATORY TO GENERATING AN ELECTOSTATIC IMAGE AND PHOTOGRAPHICALLY BLEEDING AWAY THE UNIFORM CHARGE IN SELECTIVE MANNER TO FORM THE IMAGE WHILE SHIELDING A PORTION OF THE SURFACE ENCIRCLING THE IMAGE TO RETAIN THE UNIFORM CHARGE IN SAID PORTION TO CREATE AN ELECTROSTASTIC FENCE. 